1. Field of the Invention
The present invention relates to a lithographic printing original plate. The present invention also relates to a negative type lithographic printing original plate used for direct plate making based on digital signals from computers or the like.
2. Background Art
As lithographic printing original plates, those having a lipophilic photosensitive resin layer disposed on a hydrophilic support have heretofore been used widely. A plate making method commonly employed is a method of forming a desired printing plate by carrying out mask exposure via a lith film and then dissolving and removing a non-image portion.
A digitalization technique which electronically treats, accumulates, and outputs image data by using a computer has spread widely. A variety of new image output systems suited for such digitalization techniques have therefore been put into practical use. As a result, there is a strong demand for the development of computer-to-plate (CTP) techniques in which light with high directivity, such as laser light, is scanned in accordance with digitalized image data, and a printing plate is made directly without using a lith film. It is therefore an important technical problem to obtain a lithographic printing original plate capable of satisfying such a tide.
As such a lithographic printing original plate which can be subjected to scanning exposure, that has, on a hydrophilic support, a photosensitive layer containing a photosensitive composition capable of generating an active species such as radicals by exposure to a laser, has already been proposed and is on the market. A negative type lithographic printing plate can be obtained by subjecting such a lithographic printing original plate to laser scanning exposure based on digital data to generate a chemical species, causing a physical or chemical change of the photosensitive layer to insolubilize it by making use of the action of the chemical species, and then carrying out development.
With recent innovation in CTP technology, usage frequency of highly precise FM screening has increased, and better print quality is desired. When FM screening, in which an assembly of minute images, is used for image formation is employed, however, it is very difficult to achieve improvement in print durability and chemical resistance of the minute images. For example, a method described in Japanese Application Publication No. 04-161957 is conventionally known as a method of improving print durability. Japanese Application Publication No. 04-161957 describes that the addition of a trace amount of a silane coupling agent having an unsaturated double bond to a photopolymerizable composition which is reactive with ultraviolet or visible light improves adhesion between an aluminum substrate and a photosensitive layer disposed thereover, thereby improving print durability. In such a photosensitive layer, improvement in print durability is observed in a very large image portion corresponding to from 65 to 110 lines, but improvement in print durability and chemical resistance of minute images used in FM screening has not yet been confirmed. In recent years, a method of improving print durability of a thermal negative type photosensitive composition which is reactive with light in the infrared region is disclosed in Japanese Application Publication No. 2004-109851, Japanese Application Publication No. 2007-272079, and Japanese Application Publication No. 2007-272134. These patent documents describe that a high-density and strong crosslink structure is formed promptly by using a specific alkali soluble polymer and therefore high density and excellent print durability can be achieved. Also in these patent documents, however, print durability and chemical resistance of minute images have not yet been confirmed. In particular, thermal negative type lithographic printing plates omitting a polymerization acceleration step called “preheating” is different from a printing plate using a photopolymerizable composition which is reactive with light in the ultraviolet and visible light regions so that curing of the image is insufficient at a low exposure amount, and print durability of a minute image becomes insufficient. As a result, the plate is likely to have degraded chemical resistance. For high-quality printing making use of FM screening, improvement in print durability and chemical resistance of a minute image portion become indispensable conditions, and there is therefore room for early improvement.